1. Field of the Invention
The present invention relates to an ignition timing control system and method for a variable cylinder internal combustion engine which is operated by switching an operation mode thereof between an all-cylinder operation mode in which all of a plurality of cylinders are operated, and a partial-cylinder operation mode in which part of the plurality of cylinders are deactivated, as well as an engine control unit.
2. Description of the Prior Art
Conventionally, an ignition timing control system for a variable-cylinder internal combustion engine has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H10-30477 (FIG. 1, pages 3 and 4). This engine is not of the above variable-cylinder type but of a normal type. In the engine, when an idle switch is ON, i.e. when a throttle valve is in a substantially fully-closed state, and at the same time the rotational speed of the engine is equal to or higher than a predetermined fuel cut-off speed, it is determined that deceleration operating conditions are satisfied under which fuel cut-off operation for cutting off fuel supply to the engine should be carried out. Further, when the deceleration operating conditions are satisfied, the fuel cut-off operation is not instantly started but carried out after the lapse of a predetermined delay time period. More specifically, after satisfaction of the deceleration operating conditions, the ignition timing is not retarded until a first delay time period has elapsed, and thereafter, until a second delay time period has elapsed, the ignition timing is progressively retarded. Then, only after the lapse of the second delay time period, the fuel cut-off operation is started. Thus, the ignition timing is progressively retarded before the start of the fuel cut-off operation to progressively reduce torque, whereby shock is suppressed which is caused by suddenly reduced torque resulting from the fuel cut-off operation.
As described above, according to the conventional ignition timing control system, when it is determined that the deceleration operating conditions are satisfied, the ignition timing is retarded during the delay time period before the start of the fuel cut-off so as to suppress shock caused by the fuel cut-off operation. However, when the ignition timing control system is applied to the variable-cylinder internal combustion engine, there occurs the following problem: In the partial-cylinder operation mode, since part of the cylinders are deactivated, output torque per cylinder tends to be larger than in the all-cylinder operation mode, on condition that the same torque is output by the whole engine.
As a result, when the deceleration operating conditions are satisfied, if the engine is in the partial-cylinder operation mode, it is impossible to reduce the torque sufficiently even though the ignition timing is retarded by the above method, and hence there is a fear of a sudden reduction of torque at the start of the fuel cut-off operation causing a large toque shock.
The above inconvenience can be avoided, e.g. by setting the amount of retardation of the ignition timing to a larger value. However, if the engine is in the all-cylinder operation mode when the deceleration operating conditions are satisfied, the output torque per cylinder is so small that the ignition timing retarded using the amount of retardation set as above reduces the torque more than required, resulting in occurrence of shock or low rotational speed of the engine.